This disclosure relates generally to the field of work scheduling in a mainframe operating system.
A mainframe operating system (OS) such as z/OS (a product of IBM, see http://www-03.ibm.com/systems/z/os/zos/ for more information) provides continuous, high-volume operation with high security and stability. A mainframe OS may be a 64-bit operating system that offers the functionality of modern mainframe operating systems alongside the functionality of older mainframe operating systems, allowing for backward compatibility with the older systems. A mainframe computer system may comprise a single operating system instance, or multiple operating system instances running in parallel.
The mainframe operating system may allow definition of goals and business importance for work. Work may comprise one or more related programs executing within a virtual address space. The mainframe OS may assign different priorities to different virtual address spaces in the system according to the relative importance of the work that belongs to that address space. The workload manager (WLM) of the mainframe operating system may automatically manage the various units of work based on the priority of their respective address spaces. The system processors may execute high importance work first, allocating processor time to lower importance work as the processor time is available.
Work allocated to one address space may, in the course of execution, need to perform an action that is executed in a different address space. The work may request the action be run asynchronously in the different address space. The request to run the associated program asynchronously may comprise a service request, which may be represented by a control block that may be referred to as an SRB (Service Request Block). The service request is a type of work request that allows the operating system to perform additional work, or services, on behalf of requesting work. The additional work associated with the service request may be represented by a control block that may be referred to as a work element block (WEB). A WEB may represent any work in the system, whether or not the work is initiated by a service request.
Service requests provide a mechanism for communication between address spaces. The service request may be addressed, or targeted, to a lower priority address space than the address space of the requesting work. The address space that originates the service request may be referred to as the source address space, and the address space in which the service request executes may be referred to as the target address space. The WLM may have no knowledge of the importance of a service request beyond its target address space, resulting in the work associated with the service request being scheduled for execution at the priority of the target address space. A service request may have global (system-wide) or local (address space-wide) priority. The system places the service request in a set of pending service requests, and the work associated with the service request is pending until the work associated with the service request becomes the highest priority work in the set.
A service request with global priority has a higher priority than that of any other address space, regardless of the priority of the target address space. However, a service request with local priority has a priority equal to that of the target address space. The assignment of global or local priority depends on the importance of the request; for example, a service request for an I/O interrupt may be scheduled at a global priority, to minimize I/O delays.
The target address space may be incapable of executing a local service request if the system processors are busy providing service to higher priority work. A source program may continue execution at its higher priority, and continue to issue local service requests for a lower-priority target address space if issued service requests for the target address space are not executed in a timely fashion. This may result in large numbers of service requests being issued for the target address space. The size of the set of pending service requests for the target address space may become a performance issue. It is also possible that the virtual storage available to the set of pending service requests may be exceeded, possibly resulting in failure of the mainframe system.